Blood sugar is a term used to refer to the amount of glucose in the blood. Glucose, transported via the bloodstream, is the primary source of energy for the body’s cells.

Blood sugar concentration, or glucose level, is tightly regulated in the human body.

Normally, the blood glucose level is maintained between about 4 and 8 mmol/L (70 to 150 mg/dL). The total amount of glucose in the circulating blood is therefore about 3.3 to 7g (assuming an ordinary adult blood volume of 5 liters).

Glucose levels rise after meals and are usually lowest in the morning, before the first meal of the day. Failure to maintain blood glucose in the normal range leads to conditions of persistently high (hyperglycemia) or low (hypoglycemia) blood sugar. Diabetes mellitus, characterized by persistent hyperglycemia of several causes, is the most prominent disease-related to failure of blood sugar regulation.

Although it is called “blood sugar,” sugars besides glucose are found in the blood, such as fructose and galactose. Only glucose levels are regulated via insulin and glucagon.

There are two different major methods that have been used to measure glucose. The older one is a chemical method that exploits the nonspecific reducing property of glucose in a reaction with an indicator substance that acquires or changes color on its reduction. Since other blood compounds also have reducing properties, this method can have erroneous measurements up to 5 to 15 mg/dl. This is solved by the enzymatic methods that are highly specific for glucose.